1. Field of the Invention
The present invention relates to the casings of compressors and turbines in a gas turbine engine and, more particularly, to the thermal control of horizontal split-line flanges associated with casings split into two halves.
2. Description of Related Art
In order to enhance ease of maintenance, casings for the turbines and/or compressors of a gas turbine engine are oftentimes split axially into two halves along a horizontal plane and then secured together by means of a series of bolts or other devices through a pair of conventional flanges in mating relation. This permits one half of the casing to be removed, giving access to the internal blades, vanes and shrouds, without disturbing the structural integrity of the turbine/compressor. Such design also permits maintenance to be performed on the turbine/compressor while the gas turbine engine is on wing.
A disadvantage of this arrangement, however, is that the horizontal split-line (HSL) flanges will be thermally mismatched with the casing skin. This stems from the casing interior wall being typically exposed to high velocity, high temperature engine cycle air (e.g., for both high pressure compressor and turbine casings) while the exterior horizontal split-line flanges are washed in the relatively low temperature, low velocity environment of the engine enclosure. Therefore, during an acceleration of the engine from ground idle to take-off power, the HSL flanges will be colder than the casing skin. The reverse occurs during deceleration of the engine from stabilized high power to idle conditions. The temperature gradient existing between the HSL flanges and the remainder of the casing has been known to cause the casing to ovalize since the HSL flange locations pinch inward towards a centerline of the casing. This behavior, in turn, causes rubs with the rotor blade tips in the high pressure turbine application, which leads to increased clearances and lower efficiency. In the high pressure compressor application, larger tip clearances will result in lower stall margin and poorer engine operability characteristics.
One solution to the above-described problems has been to eliminate the HSL flanges by using a 360xc2x0 casing. While the thermal mismatch is eliminated, such a design requires the rotor to be assembled simultaneously with the stator. The rotor balance operation must then be accomplished while in the stator assembly. Moreover, as detailed above, a significant maintainability issue is associated with this type of design.
Accordingly, it is desirable for a gas turbine engine casing to be developed for use with both compressors and turbines which provides the ease of access for maintenance purposes exhibited by split-line casings and minimizes any thermal mismatch between the HSL flanges and the rest of the casing.
In accordance with one aspect of the present invention, a casing for a gas turbine engine having a longitudinal axis extending therethrough is disclosed as including a first casing portion and a second casing portion. Each casing portion has a substantially arcuate section and a split-line flange extending from the ends thereof. The first and second casing portions are mated at each end by connecting together respective pairs of the split-line flanges. A channel is formed in at least one mating surface of the split-line flanges to provide an axial passage therethrough so that air flow provided to the axial passage reduces a temperature gradient between the arcuate sections and the split-line flanges of the first and second casing portions. The gas turbine engine casing also includes a first radial channel formed in at least one mating surface of the split-line flanges to provide an entrance to the axial passage and a second radial channel formed in at least one mating surface of the split-line flanges to provide an exit to the axial passage so that flow communication is established between a flowpath through the casing and the axial passage.
In accordance with a second aspect of the present invention a split-line flange for a casing portion of a gas turbine engine is disclosed where the casing has a longitudinal axis extending therethrough. The split-line flange includes a forward end, an aft end, a first side connected to an end of a substantially arcuate casing section, a second distal side opposite the first side, a first surface located between the first and second sides which mates with an adjacent split-line flange of a second casing portion, and a second exterior surface opposite the first surface. A channel is formed in the first surface generally extending from said forward end to said aft end so as to provide an axial passage when the split-line flange is mated with the adjacent split-line flange. The split-line flange further includes a first radial channel formed in the first surface to provide an entrance to the axial passage and a second radial channel formed in the first surface to provide an exit from the axial passage.